I went to a comprehensive school in the 90s/00's (supposedly a golden age for finding). As a bright child, it was utterly utterly shit. Aspirations were low, behaviour was very poor. I was bullied relentlessly, but I kept on studying because I didn't want to give up. I changed school for sixth form and got into oxbridge - no one in my school had done that for years.
(We had mixed classes for the first year and about 25% of the class could barely read, certainly not well enough to study a very simple abridged version of Oliver Twist, like an "early readers" one, that I could have read easily at 7). The TA in our class was very busy and for me that year was a waste of time in many ways - you could have given me the text books on day 1 and I could have sat at home doing them and got to the same place (except in my maths class, where my teacher was absolutely phenomenal at differentiation and took no shit).
I am the kid that MN says "well bright kids will do well anywhere", but it took a huge mental toll on me having no friends because no one would be seen dead with me and it caused me serious MH issues.
I spent a lot of my life wishing that I had had the option of going to grammar school. In fact, my sixth form pretty much was a grammar school (was a church school but basically became not religiously selective at sixth form, kicked out anyone without 5 A*/C and B in chosen subject and gave places to kids like me) - it was heaven. The kids there were so nice and all wanted to learn.
I now live in a grammar county and I suspect my kids will get into grammar school in due course (I know many people say this, obviously, but my DD in particular is very like me and very much a bright self starter). If they don't get in, I'd probably consider sending them private, certainly over one of the options.
However, I don't agree with grammar schools. What I actually needed from school could have been achieved by a combination of better aspirations, better discipline (not hardline, or stupid technical infringements, but firm/fair/consistent approach) - I didn't need a system tailored for me, just one that felt less like a jungle.
Rather than focusing on the top 25%, we should be focusing on the bottom 25%. Some of them will have SEN and should be better supported in mainstream, some may be better in a proper SEN environment (30 kids in a class is not for everyone), some will need better behavioural interventions that better account for their circumstances. And ALL of those "bottom" 25% should have something to aspire to, not the prospect of failing their GCSEs and then being written off.